Researchers from the University of Nottingham in England recently conducted a study, published in the Heart journal, to determine the effectiveness of statins, which are used to treat high cholesterol.

To do so, they examined more than 165,000 people who were taking the drug to decrease heart disease risk by lowering cholesterol levels. According to national guidelines, statins must reduce bad cholesterol levels by 40% after two years to be considered effective, the team noted.

After analyzing the results, they found 51% of the subjects had a poor response to the treatment, which put them at a higher risk of developing heart disease. The drug was effective for the other 49%.

“Our research has shown that in almost half of patients prescribed statins they are very effective and offer significant protection against cardiovascular disease,” co-author Stephen Weng said in a statement. “However, for the other half – whether it’s due to your genetic make-up, having side effects, sticking to the treatment, or other medications – we don’t see that intended benefit.”

The scientists believe genetic factors and patients not staying on their medicine could be the main reasons the medication did not work well for more than half of the participants.

They also explained the United Kingdom follows the same trends in the United States and other countries, so their findings could be applied to Western populations.

The scientists now hope to continue their investigations, so they can better predict patients’ cholesterol responses to treatments.

The authors concluded: “We have to develop better ways to understand differences between patients and how we can tailor more effective treatment for those millions of patients who are simply blanket prescribed statins.”